Developed in the field of wireless communications are systems where a radio transmission apparatus transmits packets at high speed to a communication apparatus that is a transmission destination of the packets, and particularly, downlink high-speed packet transmission systems where a plurality of mobile terminal apparatuses shares a high-speed large-capacity downlink channel, and a base station apparatus transmits packets to the mobile terminal apparatuses. Among the downlink high-speed packet transmission, as a higher packet transmission system of IMT-2000, the system called HSDPA (High Speed Downlink Packet Access) is considered for the purpose of increasing the downlink peak transmission speed, decreasing transmission delay, and achieving high throughput and the like. As techniques constituting HSDPA, disclosed in 3GPP (3rd Generation Partnership Project) is the transmission system called AMC (Adaptive Modulation and Coding) (see 3GPP TR25.848 “Physical layer aspects of UTRA High Speed Downlink Packet Access).
The AMC technique is to vary adaptive modulation parameters such as the M-ary number, error correcting coding rate and the like adaptively at high speed corresponding to variations in channel quality. In the AMC technique, as the channel quality is higher, the transmission rate can be increased by using a large M-ary number and high coding rate. More specifically, each mobile terminal apparatus measures downlink propagation path environments whenever necessary, and transmits an adaptive modulation request (CQI: Channel Quality Indicator) based on the measurement result to a base station apparatus. The CQI corresponds to a pair of adaptive modulation parameters. Based on the CQI, the base station apparatus determines a mobile terminal apparatus that is a transmission destination of transmission data, concurrently determines an optimal transmission rate, and transmits the transmission data.
Considered as the adaptive modulation parameters are the M-ary number (for example, switching between QPSK (Quaternary Phase Shift Keying) and 16QAM (Quadrature Amplitude Modulation)), coding rate (for example, performing turbo coding at R=⅓ and varying by puncturing or repetition) and the like. Meanwhile, used as the channel quality information is, for example, CIR (Carrier to Interference Ratio), SIR (Signal to Interference Ratio), transmit power of a dedicated channel (for example, DPCH (Dedicated Physical Channel)), and the like.
However, in the conventional system, when the base station apparatus erroneously receives the CQI transmitted from a mobile terminal apparatus, and transmits downlink data to the mobile terminal apparatus at a transmission rate determined based on the erroneously received CQI, the mobile terminal apparatus also erroneously receives the data because the data is transmitted at a transmission rate different from the requested one, and as a result, repeats retransmission of data, and there arises a problem of decreasing the throughput. In particular, although the mobile terminal apparatus transmits a request for a low transmission rate i.e. request for a low value of CQI (low CQI), when the base station apparatus erroneously receives the request as a high transmission rate i.e. request for a high value of CQI (high CQI), and transmits data to the mobile terminal apparatus at a high transmission rate based on the erroneous CQI, the mobile terminal apparatus does not meet the quality required of the transmitted high transmission rate, and there is a high possibility of erroneously receiving the data.